soltest.sh, EthOptions.cmake, and CODING_STYLE.md tweaks

solidity.sh:
  * introduce SOLIDITY_BUILD_DIR env var for soltest.sh,
  * add --help output

EthOptions.cmake: more complete configuration information

CODING_STYLE.md: note existence of .editorconfig
contributing.rst: note that tests are Boost C++ unit tests

scripts/soltest.sh

@@ -1,5 +1,4 @@
 #!/usr/bin/env bash
-
 set -e
 
 REPO_ROOT="$(dirname "$0")"/..
@@ -7,6 +6,27 @@ USE_DEBUGGER=0
 DEBUGGER="gdb --args"
 BOOST_OPTIONS=
 SOLTEST_OPTIONS=
+SOLIDITY_BUILD_DIR=${SOLIDITY_BUILD_DIR:-build}
+
+usage() {
+    echo 2>&1 "
+Usage: $0 [options] [soltest-options]
+Runs BOOST C++ unit test program, soltest.
+
+Options:
+  --debug                  soltest invocation prefaced with: \"$DEBUGGER\"
+  --debugger *dbg-cmd*     soltest prefaced with your own debugger command.
+  --run_test | -t  *name*  filters test unit(s) to include or exclude from test.
+                           This  option can be given several times.
+  --boost-options *x*      Set BOOST option *x*.
+  --show-progress | -p     Set BOOST option --show-progress.
+
+Important environment variables:
+
+SOLIDITY_BUILD_DIR: Sets directory under the repository root of where test/soltest should be found.
+           The default is \"${SOLIDITY_BUILD_DIR}\".
+"
+}
 
 while [ $# -gt 0 ]
 do
@@ -23,7 +43,11 @@ do
 			shift
 			BOOST_OPTIONS="${BOOST_OPTIONS} $1"
 			;;
-		-t)
+		--help)
+		        usage
+			exit 0
+			;;
+		--run_test | -t )
 			shift
 			BOOST_OPTIONS="${BOOST_OPTIONS} -t $1"
 			;;
@@ -40,4 +64,4 @@ if [ "$USE_DEBUGGER" -ne "0" ]; then
 	DEBUG_PREFIX=${DEBUGGER}
 fi
 
-exec ${DEBUG_PREFIX} ${REPO_ROOT}/build/test/soltest ${BOOST_OPTIONS} -- --testpath ${REPO_ROOT}/test ${SOLTEST_OPTIONS}
+exec ${DEBUG_PREFIX} ${REPO_ROOT}/${SOLIDITY_BUILD_DIR}/test/soltest ${BOOST_OPTIONS} -- --testpath ${REPO_ROOT}/test ${SOLTEST_OPTIONS}