Use TimerOutputs.jl to measure time and memory performance

[ahojukka5]

• See issue #98.
• Added TimerOutputs to REQUIRE file.
• JuliaFEM has now TimerOutput() handle to and function print_statistics() to get summary of time and memory usage.
• TimerOutputs.jl is used to measure performance of LinearSolver at the moment and tested in file test_elasticity_2d_linear_with_surface_load.jl

Example output:

 ──────────────────────────────────────────────────────────────────────────────
                                       Time                   Allocations
                               ──────────────────────   ───────────────────────
       Tot / % measured:            14.6s / 33.7%            623MiB / 29.8%

 Section               ncalls     time   %tot     avg     alloc   %tot      avg
 ──────────────────────────────────────────────────────────────────────────────
 assemble problems          1    3.19s  65.0%   3.19s    146MiB  78.8%   146MiB
 solve linear system        1    1.38s  28.1%   1.38s   30.6MiB  16.5%  30.6MiB
 update problems            1    177ms  3.61%   177ms   5.81MiB  3.13%  5.81MiB
 initialize solver          1    162ms  3.30%   162ms   2.85MiB  1.54%  2.85MiB
 ──────────────────────────────────────────────────────────────────────────────

[coveralls]

Coverage increased (+0.002%) to 90.173% when pulling 64250962adbdc0a8eca2e80dee06e72748c25c13 on feature/TimerOutputs.jl into 1c67f1c1f8e7198d4c30394a45d8396c87491182 on master.