Added initial source, build system and tests.
Created library source files `astr.h` and `astr.c`, adding the initial
library function `astr_alloc_empty()`.
Created test source file `astr_tests.c` with a test for
Ran `autoscan` to create initial `configure.ac` file and added
additional checks. Also added `Makefile.am` with targets to create both
`libastr` and `astr_tests`.
Updated `.gitignore` with Autotools generated files and directories.
Updated the readme with instructions to run `autoreconf` to regenerate
the build system.