Added line/col support to lexer
Revised error reporting
Revised error formatting, fixed tests
Added error context for parsing of subtrees
Added missing test entry
Revised error message in parser test
Run gofmt
Fixes #20 : Creation of subgroup in table arrays
Add 1.3 to Travis' build matrix
Added Position and Document support

TomlDocument provides an optional TOML processing path where position
informaiton is stored alongside a TomlTree.
* Added Position struct
* Revised TomlTree to contain position data
* Added tomlValue to bind positions to values
* Revised parser to emit position data
* Revised token to use new Position struct
* Added tests for new functionality
* Bugfixed table array duplicate key handling
* Applied gofmt to all code