Updated documentation for godoc.
todo.
Bug fix to terminal parsers.

INT and FLOAT parsers should interpret negative numbers as well.
parsec/examples package.

Added expr parser and json document parser as example parsers under
parsec/examples package.

Also added test cases and benchmarks for example parsers.

BenchmarkExpr1Op           50000             44601 ns/op
BenchmarkExpr2Op           50000             48421 ns/op
BenchmarkExpr3Op           50000             53592 ns/op
BenchmarkExpr              10000            103738 ns/op
BenchmarkJsonInt           50000             49436 ns/op
BenchmarkJsonFloat         50000             42681 ns/op
BenchmarkJsonString        50000             34529 ns/op
BenchmarkJsonBool         200000             13328 ns/op
BenchmarkJsonNull         100000             24868 ns/op
BenchmarkJsonArray         20000            102556 ns/op
BenchmarkJsonMap           10000            170938 ns/op
command line program.