Bugfix in reading MAT_v5 files

I just discovered Julia this afternoon and was excited to see this library for reading Matlab files.  But, when I installed it and tried reading a simple MAT-file, I got this error:

```
ERROR: no method Dict(Int64,)
 in read_struct at /home/geoff/.julia/MAT/src/MAT_v5.jl:147
 in read_matrix at /home/geoff/.julia/MAT/src/MAT_v5.jl:240
 in read_matrix at /home/geoff/.julia/MAT/src/MAT_v5.jl:215
 in read at /home/geoff/.julia/MAT/src/MAT_v5.jl:292
 in matread at /home/geoff/.julia/MAT/src/MAT.jl:74
```

I'm guessing maybe the syntax for the Dict constructor has recently changed? Anyway, it seems like changing line 147 to:

```
data = Dict{ASCIIString, Any}()
sizehint(data, n_fields+1)
```

seems to work.