support reading from postgres 'text' type
support reading from postgres 'bytea' type
fix probable bug in pgdata support for abstract Strings
support encoding other postgres 'text, 'bpchar', and 'bytea' types
tests for other pg string types and bytea
test/dbi: test text and bytea types
test/data: I just noticed julia already has C_NULL defined
fix warnings about deprecated use of (implicit) convert()

The unit tests were causing the following warning to be printed:

WARNING: convert{T}(p::Type{Ptr{T}},a::Array) is deprecated, use convert(p,pointer(a)) instead.
 in convert at deprecated.jl:26
 in PQexecPrepared at /home/srp/.julia/PostgreSQL/src/libpq_common.jl:4
 in executemany at /home/srp/.julia/PostgreSQL/src/dbi.jl:211
 in test_dbi at /home/srp/.julia/PostgreSQL/test/dbi.jl:91
 in include at ./boot.jl:245
 in include_from_node1 at ./loading.jl:128
 in include at ./boot.jl:245
 in include_from_node1 at loading.jl:128
 in process_options at ./client.jl:285
 in _start at ./client.jl:354

This fixes that.

See https://github.com/JuliaLang/julia/issues/7694 for more info
take advantage of c_free() and c_realloc() support of nullptr

free(NULL) is a no-op, it will not cause a segfault or any undefined behavior

Likewise, realloc(NULL, size) is treated as if it were malloc(size)

As such, we can avoid premallocing for param_ptrs, as pgdata() will call
realloc() if it fills in that param. This is extra beneficial for strings
as we avoid allocating a small string param only to turn around and realloc
it for a larger actual string.
Merge pull request #3 from srp/master

support reading 'text' and 'bytea' types