Add ability to load trees in different formats

[niemasd]

Currently, I allow loading trees from Newick strings, but some additional things to add:

• Directly from file using only TreeSwift methods (i.e., don't require user to load string from file manually)
• Nexus
• NeXML (this description may be more useful)

[niemasd]

Added ability to load Newick trees from file (plaintext or gzipped) in version 1.0.27

[niemasd]

Added ability to load multi-line Newick files in version 1.0.29

[niemasd]

Added ability to load trees from Nexus files in version 1.0.30

[niemasd]

Added ability to load trees from NeXML files in version 1.0.32