Flags to ignore import and package line length

The Google Java Style guide says that package and import statements
should always be on a single line even if it is over the maximum (80 or
100) character line length. This seems like a good rule to follow for
groovy too.

I added an ignorePackageStatement and ignoreImportStatement options to
the LengthLengthRule. Both default to true.
Merge pull request #37 from kyleboon/ignore-import-line-length

Thanks for the contribution!