Added --tmpdir option to set the temporary directory

With either --tmpdir or EASYBUILD_TMPDIR you can set the directory used
for temporary storage.
Check if temp storage allows to execute files

If your temp storage is mounted with 'noexec', this can cause problems
for certain easybuilds (numpy). We now print a warning about this.
Use random dir in $EASYBUILD_TMPDIR to build.

Should solve conflicts with multiple users using EB on the same machine.
Merge branch 'settmpdir' of github.com:wpoely86/easybuild-framework into settmpdir

Conflicts:
	easybuild/tools/options.py
move functionality required to set tmpdir and check for noexec to set_tmpdir function in tools/config.py, add unit test for set_tmpdir
add missing import
Return EB tmpdir in main
Check return value in settmpdir unit test
stop fidling about in home dir in test_set_tmpdir, restore original environment and reset tempfile.tempdir after every test
fix typo
restore original environment and reset tempfile.tempdir after toy unit tests
add unit test for --tmpdir, restore environment and reset tempfile after each call to main()
Merge branch 'develop' into settmpdir

Conflicts:
	easybuild/main.py
fully rely on modify_env to restore original environment for tests
Merge pull request #695 from wpoely86/settmpdir

/tmp mounted with noexec gives trouble