Join libeuler and main
Rename main => euler
Rename directory 0001-0050 to prob0001_0050
Rename struct and enum
Move problem definition into subdirectory
Add option to solve specified problem
Change solver return values: uint -> ~str
Move solve function to euler.rs
Move all problem info into probXXXX.rs
Add prob0051_0100 module
Reform prime
Show computation time
Set --opt-level 3
Inlining prime
Add 'make debug' and 'make release' (debug is default)
Add problem 51