Implement native 64 bit values and data.table support

[DavZim]

Closes #16

• changes all std::vector<> to Rcpp::XXXVector.
• if needed, values (>= 6 byte) are stored in integer64 vectors
• the returning data.table is directly created in cpp (following https://github.com/lsilvest/data.table_at_c_level)

The only thing that isn't done in Rcpp directly is creating the datetime column (nanotime), as this requires the date conversion in cpp as well.

This PR reduces computing time (approx. times for only one run...)

• count_messages(file) 0.36 sec to 0.36
• get_orders(file) 4.4 sec to 2.65
• get_trades(file) 0.86 sec to 0.87
• get_modifications(file) 2.97 sec to 1.98