FreeBSD support

[Arc-blroth]

Tested this in a FreeBSD VM in Virtualbox and everything seems to be good - hopefully my afternoon perusal of FreeBSD docs wasn't too bad...

Fixes #9

[Arc-blroth]

@yurivict If it isn't too much of a hassle for you, would you mind testing this PR on an actual FreeBSD system? (cargo test should pass)

[yurivict]

Tests passed:

$ cargo test
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.05s
     Running unittests src/lib.rs (target/debug/deps/memory_stats-d7171516108cde26)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/allocate_4mb_of_memory.rs (target/debug/deps/allocate_4mb_of_memory-fcb7e1ecdbbabe36)

running 1 test
test allocate_4mb_of_memory ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.34s

   Doc-tests memory-stats

running 1 test
test src/lib.rs - (line 25) ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.11s