Rewrote lib/gpt

- Split up header & table parsing
- Removed dead code / unneccessary parts
- Changed API
Updated lib/partition: Adjusted to changed API
Deleted lib/partition-table: Obsolete
Updated test/main: Adjusted to API changes
Deleted lib/partition-types: Obsolete
Updated lib/gpt: Added more options
Updated README: Added some usage information
Merge pull request #2 from jhermsmeier/next

Rewrite GPT: Decouple header and partition table parsing