Allow project-specific java parameters.

Summary:
We need to increase the heap size for Buck.  This lets projects add
java runtime flags.

Test Plan: unit tests; run a command that OOMs, add .buckjavaargs, verify success

bin/buck_common

diff --git a/bin/buck_common b/bin/buck_common
index 32b5c96..b67c0a3 100755
--- a/bin/buck_common
+++ b/bin/buck_common
@@ -240,6 +240,13 @@
 ${BUCK_DIRECTORY}/third-party/java/xz-java-1.3/xz-1.3.jar:\
 "
 
+BUCK_PROJECT_JAVA_ARGS_FILE="${PROJECT_ROOT}/.buckjavaargs"
+if [[ -r ${BUCK_PROJECT_JAVA_ARGS_FILE} ]]; then
+  BUCK_PROJECT_JAVA_ARGS=$(cat ${BUCK_PROJECT_JAVA_ARGS_FILE})
+else
+  BUCK_PROJECT_JAVA_ARGS=
+fi
+
 BUCK_JAVA_ARGS="\
 -XX:MaxPermSize=256m \
 -Xmx1000m \
@@ -259,6 +266,7 @@
 -Dbuck.dx=${BUCK_PATH_TO_CUSTOM_DX} \
 -Dbuck.buckd_dir=${BUCKD_DIR} \
 -Dlog4j.configuration=file:${BUCK_DIRECTORY}/config/log4j.properties \
+${BUCK_PROJECT_JAVA_ARGS} \
 ${BUCK_EXTRA_JAVA_ARGS}"
 
 # To debug BUCK, set BUCK_DEBUG_MODE in your environment to anything non-empty,