Adds the basic header with simple information

Fixes #10 (closed) (at least for the gridded searches)

This adds a basic header with the data, username, and hostname information. Example output (from examples/transient_examples/short_transient_search_gridded.py)

# date:2017-12-20 11:54:51.736768;user:greg;hostname:greg-len-ubuntu
# _ _ F0 F1 F2 Alpha Delta
1.000000000000000000e+09 1.000172800000000000e+09 2.999998991056105524e+01 -1.000000000000000036e-10 0.000000000000000000e+00 5.000000000000000000e-01 1.000000000000000000e+00 4.206904220581054688e+01 1.000097200000000000e+09 1.980000000000000000e+04
1.000000000000000000e+09 1.000172800000000000e+09 2.999999011234983470e+01 -1.000000000000000036e-10 0.000000000000000000e+00 5.000000000000000000e-01 1.000000000000000000e+00 4.204999923706054688e+01 1.000097200000000000e+09 1.980000000000000000e+04

@dkeitel Am I missing anything from the header?