made tests use ProcessBuilder instead of List#execute() with 'env' command

it's because ProcessBuilder handle environment variables much better than List#execute()
changed to restart groovyserver for each type of integrationTest

the feature of propagation of environment variables permanentally affects groovyserver proceess.
So if the same server process is used for each type of integrationTest, problems of it may be missed.
(Actually it was missed.)
fixed a bug of a propagation of environment variable of groovyclient.sh
fixed a failing test on cygwin in windows

only with environment variables, command line is split by white space thoroughly.
fixed a bug which occurs when env value includes backslash

Sympton:
- escape characters are gone at env value
- a backslash at a bottom of env value results in combinating the line with next env line
upgraded gradle and wrapper to 1.6
fixed to include README.md in archive file
fixed an error which occurs when GROOVYSERV_WORK_DIR is a path including white spaces
version 0.12