Fix a SEGFAULT in GetTranslatedPhonemeString.

When running any of the following:

    espeak -x "[[]]"
    espeak --ipa "[[]]"

eSpeak would SEGFAULT. This is because phon_out_buf is NULL as no
phonemes have been written and as this is the first call to
GetTranslatedPhonemeString, the phoneme buffer has not been
allocated yet. However, GetTranslatedPhonemeString attempts to add
a NULL character to the end of the phoneme buffer, and thus crashes.
ph_english: use /i/ as the IPA transcription of i

The i phoneme is used for the reduced i sound in e.g. happy
(/h'api/). Thus, the following words contrast:

    word   eSpeak   IPA
    happy  /h'api/  /hˈapi/
    been   /b'i:n/  /bˈiːn/
    bin    /b'In/   /bˈɪn/

Before this change, /h'api/ would be transcribed in IPA as /hˈapɪ/
which is wrong. After this change, it is transcribed correctly.
README.md: Improve the build instructions and add a section on building/adding voices.
src/wave.cpp: fix building when portaudio.h is missing
Merge branch 'master' into android

Conflicts:
	README.md