chore(build): Added --pre-release flag

Added `--pre-release` flag so that build version includes timestamp only in case of a pre-release. By default timestamp will be omitted in the version property of `dist/bower.json`.

e.g.:
`grunt --pre-release`

When publishing a package to bower the package version string must be equal to the tag name for that package. As usually no pre-releases are getting released for this package the pre-release flag is false by default.

`(warning) bower mismatch      Version declared in the json (0.5.8-1407241001) is different than the resolved one (0.5.8)`

Sorry, didn't notice before that bower throws such warning