Added switches to the generate script

Added two switches to the generate script:

  -k (or --keys), which forces the creation of new keys.
  -d (or --dry-run), which skips the actual writing or copying phases

The second option might be useful in the future, and helped me debug the
optparse module, so I left it there.
Minor cosmetic edits to schema.py.
Minor cosmetic edits to sig.py and util.py.
Minor cosmetic edits to formats.py
Merge branch 'generate_switches' of https://github.com/SantiagoTorres/tuf into SantiagoTorres-generate_switches
Move optparse import statement.

Organize import statements of standard Python and TUF modules into separate blocks.
Merge branch 'SantiagoTorres-generate_switches' into develop
Merge pull request #225 from vladimir-v-diaz/develop

Santiago's #224 pull request (with minor edit) and cosmetic edits to miscellaneous modules